I suspect he was fiddling with it.
1) He cocked the pistol and held the hammer in the cocked position with his thumb. Remembering he didn't need to take a shot, he thought, "I have to drop the hammer," and pulled the trigger. The hammer probably moved forward ever so slightly held by his thumb. He got distracted moving the hammer forward and backward, fiddled some more, didn't twig the trigger was back, forgot he had pulled the trigger, and let the hammer go.
When I am decocking a single-action pistol, I always hold the trigger back as a reminder that the trigger has been pulled.
2) Of course, he could have been fiddling, let the hammer go with it not having reached full cock, and it wasn't caught by the half-cock notch (For it not to catch is quite common on this particular model of gun. Something for the user to be aware of, and not a fault as such. The half-cock notch was not designed as a safety; it is too shallow to act as such).
If this is what happened, he wouldn't have needed to pull the trigger. For safety, the hammer should have been resting on an empty chamber. If it was, option two could not have happened as the pistol only indexes on the next chamber when fully cocked. If not fully cocked, the cylinder moves back to index on the empty chamber as the hammer falls.Reason for so many edits: Trying to make it easy to understand how a single-action Navy works.
I give up here's a video.
"All issues are political issues, and politics itself is a mass of lies, evasions, folly, hatred and schizophrenia" Orwell
E l/r -10 : L/A -7.64